In flexographic printing, also known as relief printing, ink is transferred from a pool of ink to a substrate by way of a printing plate. The surface of the plate is shaped so that the image to be printed appears in relief, in the same way that rubber stamps are cut so as to have the printed image appear in relief on the surface of the rubber. Typically, the plate is mounted on a cylinder, and the cylinder rotates at high speed such that the raised surface of the printing plate contacts a pool of ink, is slightly wetted by the ink, then exits the ink pool and contacts a substrate web, thereby transferring ink from the raised surface of the plate to the substrate to form a printed substrate.
Flexographic printing competes with other forms of printing, e.g., lithography, gravure and letterpress printing. Those involved in the flexographic printing industry are constantly striving to improve the flexographic printing process in order to more effectively compete with other printing methods. One area which has received much attention from researchers is the development of improved plates for flexographic printing.
A common approach currently used to make flexographic printing plates utilizes a photopolymerizable, also known as a photosensitive, photoimageable, photopolymer, photohardenable or photocurable, resin composition. While many different photopolymerizable resin compositions are known, they share the quality that upon exposure to certain wavelengths of light, the resin reacts with itself to form a structure that is insoluble in ink. Thus, photopolymerizable resin compositions may be used to form a hard, ink-insoluble, raised surface for photopolymer plates.
To prepare a printing plate with typical commercially available equipment, an image-bearing transparency or negative, i.e., a transparent film having opaque regions corresponding to the reverse of the image which one desires to impart to a printing plate, is placed on a glass platen, and covered with a transparent, polymeric coverfilm such as polypropylene. The transparency and coverfilm are secured by vacuum to the platen, and a layer of photopolymerizable resin is placed on the coverfilm while simultaneously laminating a backing sheet, also sometimes called a substrate, to the top of the resin. Then actinic radiation is shined through the glass platen toward the backing sheet. The regions of the resin which are impinged by the actinic radiation undergo polymerization to form an insoluble structure. The polymerization of the resin is known as curing, and the product of the polymerization is known as a cured resin.
The regions of the resin layer which were protected from the actinic radiation by the opaque regions of the transparency are washed away using developer solution, in a step called the development step. The cured regions are insoluble in the developer solution, and so after development a relief image formed of cured photopolymerizable resin is obtained. The cured resin is likewise insoluble in certain inks, and thus may be used in flexographic printing as described above. U.S. Pat. No. 2,760,863 to Plambeck describes another typical process for preparing a printing plate using photopolymerizable resin, wherein the image-bearing transparency is placed above the photopolymerizable layer, rather than underneath the coverfilm.
A common variation on the above-described process is to expose a liquid photopolymerizable resin to actinic radiation from two sides of the resin layer. See, for example, U.S. Pat. No. 3,848,998 to Yonekura et al. The recognized advantages of exposing from the back (through a backing sheet) as well as the front (through an image-bearing transparency) include better adhesion of the photopolymeric composition to the backing sheet, better relief image formation, and overcoming the inhibition to polymerization of photopolymerizable resin that is exposed to oxygen with increased control over the relief image height.
Flexographic printing plates desirably work under a wide range of conditions. For example, they should be able to impart their relief image to a wide range of substrates, including cardboard, coated paper, newspaper, calendared paper, and polymeric films such as polypropylene and the like. Importantly, the image should be transferred quickly and with fidelity, for as many prints as the printer desires to make. In consequence, and as evidenced by, e.g., the patent literature, considerable attention has been paid to the development of photopolymerizable resin compositions which have properties tailored to the specific end-use for the plate.
One substrate for which the development of flexographic printing plates has proved particularly troublesome is cardboard and other like substrates that have an uneven surface. With such substrates, the printing plate must be flexible so that it will conform to the uneven surface and evenly deliver a coating of ink thereon. However, if the plate is too soft or flexible, the image on the plate will distort under the pressure used to contact the plate with the substrate, and thus will not transfer the image with the desired fidelity. Printing plates which have been described in the prior art as being suited for uneven substrates are known as compressible printing plates.
Several approaches have been reported for the preparation of a compressible printing plate. In one approach, the plate is designed to give a low durometer reading. This approach, however, leads to an undesirable growth of the characters under the required printing impression pressure, particularly when printing on rough or uneven stock or on presses with uneven impression and/or plate cylinders.
Another commonly used technique is to adhere a layer of a compressible foam material to the back of a previously formed printing plate. In this way, one can use a fairly hard, i.e. high durometer, plate which will not provide a distorted image, and take advantage of the compressibility of the foam backing to allow the plate to bend and flex, and thereby contact all regions of an uneven substrate. A persistent problem with this approach is that it is very difficult to position the foam materials to the back of the plate. Typically, an adhesive is positioned between the plate and the foam. However, it is hard to apply the adhesive uniformly, and foam materials exhibit the problem that they stretch during mounting to the plate and, if stabilized, cause buckling when the plate is flexed.
Another technique for compressible plate manufacture is disclosed in European Patent Application No. 82300478.3, assigned to Uniroyal, Inc (the Uniroyal process). According to the Uniroyal process, a compressible foam layer is adhered to the base area of a polymer layer. The adhesion is preferably accomplished with a highly tacky adhesive or adhesive tape, however other forms of bonding such as urethane, epoxy, cyanoacrylate, anaerobic or hot melt bonding are not excluded. In addition to the use of an adhesive, the Uniroyal process is awkward because a layer of solid photopolymer material must be pre-back-exposed to light before it is laminated to the foam layer, thus involving a multiplicity of steps and not allowing variability in the thickness of the hardened layer.
More recently, U.S. Pat. No. 4,582,777 to Fischer et al. has disclosed a compressible printing plate that can be made using a liquid photopolymer composition. According to Fischer, a base layer, which may be metal or polymer film, is joined to the bottom of a compressible layer, which may be foam, with an adhesive or tie coat layer therebetween. The top of the compressible layer is also covered with a tie coat layer such as polyvinylidene chloride. A liquid photopolymer composition is then coated onto the tie coat layer, and exposed to ultraviolet radiation to cure the resin. In Fischer, a tie coat layer is disclosed as being necessarily positioned between the compressible and photocurable layer, and thus direct bonding, i.e., bonding without any intermediate material, is not achieved between the compressible material and photocurable resin of Fischer.
U.S. Pat. No. 5,006,447 to Umeda et al. comments on compressible plates formed from a foam layer, an intermediate adhesive layer and a layer of photosensitive resin. Umeda et al. report that materials present in a layer of cured photosensitive resin may transfer to the surface of a cellular foam layer and thereby reduce the adhesion between the cured photosensitive resin and the adhesive layer on the foam layer. Umeda et al. disclose a process wherein adhesive can successfully be used to adhere a foam substrate to a light-exposed photosensitive resin layer.
The most common means currently used to achieve a compressible printing plate is to tape or otherwise adhere a compressible backing sheet to a cured, image-bearing photocured resin composition. As detailed above, this approach is problematic. There is a need in the art for a simple, easily prepared, compressible photopolymer plate that overcomes the shortcomings of plates made according to prior art methodology.